VIEWPOINT pubs.acs.org/acscatalysis
Chemical Synthesis of Complex Molecules Using Nanoparticle Catalysis Huan Cong and John A. Porco, Jr.* Department of Chemistry and Center for Chemical Methodology and Library Development (CMLD-BU), Boston University, Boston, Massachusetts 02215, United States
1. INTRODUCTION The emerging field of nanoparticle catalysis represents a burgeoning area with increasing use in chemical synthesis,14 as a recent SciFinder search shows exponential growth of literature reports during the past decade (Figure 1).5 This rapidly expanding research subject has been built upon fruitful multidisciplinary collaborations between scientists in nanosciences and organic chemistry as well as the remarkable catalytic activities and selectivities of nanometric materials. Such features closely relate to the intrinsic size and shape-dependent properties6,7 of nanoparticles, which may be subtly tuned by selection and modifications of preparation conditions, support materials, and capping agents. In addition, although widely utilized homogeneous catalysts remain as synthetic chemists’ most powerful resources, heterogeneous, nanoparticle catalysis features the possibility for catalyst recycling, continuous processing, and ease of separation,8 thereby offering green and cost-effective alternatives. Despite the aforementioned promising attributes, nanoparticle catalysts have not yet found broad applications in complex molecule synthesis,9,10 with most applications thus far limited to cross-couplings11 and oxidations/reductions.12 In this Viewpoint article, we outline select examples employing nanoparticle catalysts to create new chemical bonds, with a special emphasis on applications and opportunities in complex molecule synthesis.
Figure 1. Publications on nanoparticle catalysis from 1994 to 2010.
Scheme 1. AgNP-Catalyzed Coupling of Alcohols
2. REPRESENTATIVE APPLICATIONS OF NANOPARTICLE CATALYSIS IN ORGANIC SYNTHESIS 2.1. Development of Tandem Reactions Based on Nanoparticle-Catalyzed Transformations. A number of notable
Scheme 2. Sequential Oxidation/Michael Additions of Allylic Alcohols
studies have established nanoparticle-catalyzed cascade reactions, demonstrating an efficient approach to rapidly synthesize complex molecules from simple starting materials. The development of such tandem processes has also considerably broadened the utility and versatility of nanoparticle catalysis. For example, oxidations of alcohols to carbonyl compounds catalyzed by metal nanoparticles have been extensively investigated in the literature12 and have been frequently employed in tandem reaction processes. Shimizu and co-workers (Scheme 1)13 developed redox couplings of primary and secondary alcohols 1 and 2 catalyzed by alumina-supported silver nanoparticles (AgNPs) in combination with a mild base (cesium carbonate) to afford ketones 3. The AgNP catalyst served dual roles in this highly creative tandem process, which entailed AgNP-catalyzed dehydrogenation of alcohols14,15 1 and 2 in the absence of molecular oxygen, basemediated ClaisenSchmidt condensation of the derived carbonyl compounds 4 and 5, and AgNP-mediated hydrogenation of the resulting α,β-unsaturated ketones 6. Alcohols bearing α-aryl or r 2011 American Chemical Society
heteroaryl groups generally afforded higher yields than alkylsubstituted substrates, likely because of the inherent limitations Received: September 28, 2011 Published: November 22, 2011 65
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Scheme 3. Select Examples of Nanoparticle Catalysis from the Recent Literature
containing orthogonal components should find broad applications in the synthesis of complex molecules. 2.2. Conversion of Homogeneous to Heterogeneous Reactions Employing Nanoparticle Catalysis. Another productive strategy for development of nanoparticle catalysis has taken advantage of the homogeneous transition metal catalysis. Reactions known to be catalyzed by transition metal complexes can also be facilitated by metal nanoparticles containing the same metal. This hypothesis has been proven true in many cases, as highlighted in Scheme 3 with direct comparisons of nanoparticles and transition metal catalysts made for each reaction. In addition, heterogeneous nanoparticle catalysts have exhibited the capability of offering superior catalytic activities through artful design and modifications. It should be noted, nonetheless, that minimizing the possibility of nanoparticle leaching has become increasingly crucial to establish truly heterogeneous catalysis.19 In situ-generated nanoparticles in solution have shown remarkable catalytic properties in complex molecule synthesis. For example, the Wolff rearrangement has been conventionally initiated by thermal energy, UV light, and transition metal catalysis, including silver(I) salts.20 Sudrik and co-workers (Scheme 3a)2123 found that AgNPs also served as an effective catalyst for rearrangement of diazo compounds 11 to form ketene intermediates 12,
of the ClaisenSchmidt condensation. In addition, the supported AgNP catalyst displayed a high turnover number (TON) of up to 800 and remained active after two recycles. Kobayashi and co-workers (Scheme 2)16 demonstrated a tandem aerobic oxidationMichael addition process involving oxidation of allylic alcohol 8 followed by 1,4-addition of 1, 3-dicarbonyl derivative 7 to Michael acceptor 9. This multicatalytic reaction17 was built upon authors’ earlier studies on polymer incarcerated gold nanoparticles (AuNPs)18 and a polymer/ carbon black-incarcerated boron catalyst that promotes alcohol oxidation and Michael addition, respectively. The latter catalyst was prepared from NaBH4 and PEGylated polystyrene to afford an immobilized boron species which was characterized as a tetravalent borate. The addition of porous carbon black as a support material optimized the catalyst preparation by reducing polymer cross-linking. Accordingly, a hybrid, heterogeneous catalyst containing polymer-incarcerated, bimetallic Au/Pd nanoclusters and boron was created and showed excellent dual catalytic activities after six recycles. The new catalyst also exhibited a broad substrate scope, including cyclic, aliphatic, and aromatic-substituted starting materials. Since the heterogeneous support materials may minimize possible interactions and side reactions among multiple catalysts, the development of hybrid, supported catalysts 66
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Scheme 4. AgNP-Catalyzed Oxidative Coupling of Phenols
Scheme 5. AuNP-Catalyzed Synthesis of Aromatic Azo Compounds
which further reacted with water to afford carboxylic acids 13. The catalytically active AgNPs were generated in situ by chemical (e.g., triethylamine) or electrochemical reduction of silver(I) precursors. Another example by Han and co-workers (Scheme 3b)24 showed that both in situ-generated AuNPs and homogeneous AuCl32527 catalyzed tandem alkyne activation/benzopyrylium formation/[3 + 2] dipolar cycloaddition of 14 to construct natural product-like polycycle 15 in a single step. In both studies, nanoparticles were unambiguously characterized by a number of instrumental analyses, including UVvis spectroscopy, powder X-ray diffraction, and transmission electron microscopy. However, using in situ-generated nanoparticles as catalysts has complicated accurate evaluations of the reactions’ heterogeneity and nanoparticles’ activity in terms of catalyst loading and TON. In light of the notable reaction complexity achieved in their studies, future investigations are warranted on related reactions employing supported nanoparticle catalysts. In 2001, Chung and co-workers (Scheme 3c),28 reported PausonKhand reactions catalyzed by cobalt nanoparticles (CoNPs). Building upon earilier studies using mesoporous silica and charcoal-supported cobalt catalysts,29,30 the CoNP catalyst exhibited improved activity for the intramolecular Pauson Khand reaction of enyne 16 under a pressurized CO atmosphere. In addition, four catalyst recycles were conducted without particle agglomeration or loss of activity. Extending the CoNP catalyst to intermolecular reactions between norbornadiene and terminal alkynes afforded near quantitative yields. Toste, Somorjai, and co-workers (Scheme 3d, e)31 recently reported the design and synthesis of electrophilic platinum nanoparticles (PtNPs), which were capped with either polyamidoamine dendrimers or polyvinylpyrrolidone (PVP) and further deposited onto mesoporous silica. The supported PtNPs initially showed no catalytic activity until the authors creatively devised an oxidative treatment of the nanoparticles employing a hypervalent iodine-based oxidant (PhICl2). This mild oxidant selectively oxidized more than 25% of the Pt(0) atoms in the supported nanoparticles to higher oxidation states, as confirmed by X-ray photoelectron spectroscopy (XPS), thereby converting the PtNPs into highly active electrophilic catalysts. This novel heterogeneous catalyst enabled a range of cyclization reactions, including the hydroarylation of alkyne 1832 and hydroamination of allene 20 via π-bond activation. In addition, the heterogeneity of the oxidized PtNP catalyst was unambiguously established by the “filtrate transfer” test.33 This work is of particular impact because of the crafty postsynthetic modifications that
induced the desired properties onto otherwise inactive nanoparticle catalysts.34 Inspired by an AuCl3-catalyzed intermolecular benzannulation between o-alkynylbenzaldehydes and alkynes,35 Garcia, Corma, and co-workers (Scheme 3f)36 developed a reusable cerium oxidesupported AuNP catalyst that is now commercially available. The catalyst efficiently facilitated the formation of naphthalene 24 from precursors 22 and 23 in near quantitative yield37 and was validated to be heterogeneous by both the “filtrate transfer” test33 and elemental analysis of the reaction solution. A series of indepth control experiments led to interesting observations that product 24 was obtained in 94% yield using the optimal catalyst containing 4.5 wt % (wt %) of AuNPs supported on CeO2, in comparison with only 67% yield with 1.8 wt % AuNP/CeO2 (both reactions were conducted under the otherwise same conditions with 5 mol % catalyst loading based on the amount of AuNPs). The same trends were also observed employing other support materials. The authors reasoned that most support materials can also function as adsorbents of organic molecules. Therefore, a higher amount of support materials was added to reactions when using catalysts with lower AuNP content, which caused decreased yields. 2.3. Exploration of Novel Activities of Nanoparticle Catalysts. In contrast to the homogeneous catalysis that has been extensively applied to a wide range of reaction types,38 the identification of new catalytic activities of nanoparticles is highly underdeveloped. A brief survey of known nanoparticle-catalyzed transformations1 has revealed that most of these reactions involve redox processes, which is echoed by the distinctive electrochemical properties of nanoparticles, including size and shapedependent redox potentials6 and multiple available oxidation states.39,40 On the basis of these promising characteristics, nanoparticle catalysis should offer broad opportunities for mining new chemical transformations; in particular, those that enable the synthesis of otherwise challenging complex molecules. Chattopadhyay and co-workers (Scheme 4)41 reported oxidative couplings of phenols catalyzed by chitosan-supported AgNPs. In the presence of molecular iodine as an oxidant, unsubstituted phenol (25) underwent an iodine-incorporated, oxidative CC coupling to afford biphenyl 26 in 75% yield. The products of the AgNP-catalyzed oxidative couplings were highly regioselective and substrate-dependent, with α- (27) and 67
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Scheme 6. AgNP-Catalyzed DielsAlder Cycloaddition of 20 -Hydroxychalcones
Scheme 7. Mechanistic Evidence and Proposed Mechanism for AgNP-Catalyzed Diels-Alder Cycloadditions
β- (29) naphthols affording exclusive 2,20 - (28) and 3,30 - (30) binaphthols, respectively, under the same conditions. The formation of binaphthol 30 differed in the regioselectivity for oxidative coupling of 29 using homogeneous catalysis.4246 These intriguing observations implied subtle mechanistic variations related to specific phenol starting materials, which warranted further investigation. In addition to the above notable features, the novel AgNP-catalyzed reactions showcased low catalyst loading and mild reaction conditions. Aromatic azo compounds have broad industrial applications, including drugs, dyes, and food additives.47 Corma, Garcia, and co-workers (Scheme 5)48 developed a catalytic, oxidative synthesis of azobenzenes directly from anilines, which represented a major improvement in comparison with traditional synthetic methods.4951 Employing the commercially available titanium dioxide-supported AuNPs as catalyst and molecular oxygen as terminal oxidant, a number of symmetrically substituted azobenzenes were prepared in >90% yield, regardless of the electrondonating or -withdrawing substitutions of the starting materials. Notably, the reaction between equimolar electron-rich (31) and electron-poor (32) anilines afforded unsymmetrical product 33 (methyl orange) in 91% yield in a single step. The proposed reaction mechanism entailed AuNP-mediated single-electron transfer (SET) of aniline 34 to generate radical cation 35, followed by formation of intermediate 36 through the coupling between 35 and aniline 34. Key supporting evidence included electron paramagnetic resonance (EPR) data for radical trapping of 35.52 Subsequent AuNP-mediated, stepwise oxidative electron transfer processes afforded hydrazine 37 and, finally, the desired azobenzene 38. Porco and co-workers (Scheme 6)53 recently demonstrated the utility of silica-supported AgNPs as an active catalyst for DielsAlder cycloadditions of 20 -hydroxychalcones, a biomimetic and straightforward access to the core structure shared by over 50 biologically active natural products.54 In this study, the authors accomplished the total synthesis of panduratin A (42), establishing the first example of a natural product synthesis employing nanoparticle catalysis. The key AgNP-catalyzed cycloaddition step was carried out using 0.5 mol % catalyst loading with chalcone 39 and the acid-labile trans-β-ocimene (40) in 85% yield and 19:1 d.r., thereby showcasing the catalytic activity and selectivity that has not been achieved by other methodologies.55 Final acetate deprotection of cycloadduct 41 afforded the natural product 42 in 87% yield. In addition, a series of mechanistic studies were conducted, including EPR measurements of radical trapping (confirming SET processes between AgNPs and 20 -hydroxychalcones) and
Scheme 8. AuNP-Catalyzed Synthesis of Dihydrooxadisiloles via Oxidative, Formal [3 + 2] Cycloadditions
deuterium-labeling experiments between chalcone 43 and deuterium-labeled diene 44 (Scheme 7a), supporting a concerted cycloaddition mechanism. These results suggested a working mechanism (Scheme 7b) starting with proton removal and SET from the adsorbed chalcone 43 to AgNPs. The resulting AgNPstabilized phenoxyl radical 46 and its resonance form, 47, can further undergo concerted cycloaddition with diene 48 to generate intermediate 49, which afforded cycloadduct 51 via back electron transfer, protonation, and product desorption. Stratakis and co-workers (Scheme 8)56 have reported a AuNPcatalyzed dehydrogenative, formal [3 + 2] cycloaddition between tetramethyldisiloxane 52 and acetylene 53, which was conducted at room temperature with 0.3 mol % catalyst loading, leading to the production of dihydrooxadisole 54 in 95% isolated yield. The substrate scope of the methodology was established employing a range of functionalized aromatic, aliphatic, and internal alkynes. Generally, most reactions were completed within an hour, regardless of the electron-donating or -withdrawing substitutions of the alkynes. In comparison, starting materials 52 and 53 underwent alkyne hydrosilylation catalyzed by homogeneous t-Bu3Pmodified Pt(0) catalyst,57 thereby highlighting the unique catalytic activity of the AuNPs. 68
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3. SUMMARY AND FUTURE OUTLOOK In this Viewpoint article, we have briefly summarized the utility of nanoparticle catalysis in organic synthesis through a critical overview based on recent literature examples. With increasing research endeavors being devoted to this field, new methodology development based on nanoparticle catalysis is emerging as a valuable addition to chemists’ existing tools for complex molecule synthesis. As outlined in this article, an evaluation of the possibility of catalyst leaching is crucial for the investigation of supported nanoparticle catalysts. Current studies employ mainly the “filtrate transfer” test,33 elemental analysis of the reaction solution, or both to determine catalyst leaching. In addition to these indirect methods, real-time monitoring of catalyst surface using in situ X-ray absorption spectroscopy techniques provides unambiguous proof of catalyst heterogeneity.5860 Although there is no strict rule for the nanoparticle-catalyzed reactions, the absence of evidence against catalyst leaching may lead to questioning of reaction heterogeneity as well as the identity of actual catalytic species. Future development of nanoparticle catalysis may also significantly benefit from further understanding of reaction mechanisms that will ultimately require interdisciplinary collaborations; for example, between organic chemists and material scientists. Such mechanistic studies may include experiments on reaction kinetics, theoretical modeling, catalyst surface characterization, and validation of reaction intermediates. In the future, we anticipate further discovery of new nanoparticle catalysts, including bimetallic and hybrid systems,61,62 and their applications in complex molecule synthesis in an effort to identify novel chemical reactions and tandem reaction sequences. Such opportunities should offer a strong stimulus to the continuing efforts in this vigorously advancing area.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support from the NIH (GM-073855) and AstraZeneca (graduate fellowship to H.C.) is gratefully acknowledged. H.C. thanks the NIST and ACS for sponsorship of 2011 Kenneth G. Hancock Memorial Award. We also thank Neil Lajkiewicz and Yuan Xiong for careful proofreading of the manuscript. ’ REFERENCES (1) Astruc, D. Nanoparticles and Catalysis; Wiley-VCH: Weinheim, 2007. (2) Corma, A.; Garcia, H. Chem. Soc. Rev. 2008, 37, 2096. (3) Della Pina, C.; Falletta, E.; Prati, L.; Rossi, M. Chem. Soc. Rev. 2008, 37, 2077. (4) Shiju, N. R.; Guliants, V. V. Appl. Catal., A 2009, 356, 1. (5) Based on the research topic search “nanoparticle and catalyst”. (6) Henglein, A. Chem. Rev. 1989, 89, 1861. (7) Schmid, G. Chem. Rev. 1992, 92, 1709. (8) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (9) Patil, N. T. ChemCatChem. 2011, 3, 1121. (10) Myers, V. S.; Weir, M. G.; Carino, E. V.; Yancey, D. F.; Pande, S.; Crooks, R. M. Chem. Sci. 2011, 2, 1632. (11) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. 69
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